No tolls or operating charges whatever shall be levied upon or collected from any vessel, dredge, or other water craft for passing through any lock, canal, canalized river, or other work for the use and benefit of navigation, now belonging to the United States or that may be hereafter acquired or constructed; and for the purpose of preserving and continuing the use and navigation of said canals and other public works without interruption, the Secretary of the Army, upon the recommendation of the Chief of Engineers, United States Army, is authorized to draw his warrant or requisition, from time to time, upon the Secretary of the Treasury to pay the actual expenses of operating, maintaining, and keeping said works in repair, which warrants or requisitions shall be paid by the Secretary of the Treasury out of any money in the Treasury not otherwise appropriated: Provided, That whenever, in the judgment of the Secretary of the Army, the condition of any of the aforesaid works is such that its entire reconstruction is absolutely essential to its efficient and economical maintenance and operation as herein provided for, the reconstruction thereof may include such modifications in plan and location as may be necessary to provide adequate facilities for existing navigation: Provided further, That the modifications are necessary to make the reconstructed work conform to similar works previously authorized by Congress and forming a part of the same improvement, and that such modifications shall be considered and approved by the Board of Engineers for Rivers and Harbors and be recommended by the Chief of Engineers before the work of reconstruction is commenced: And provided further, That nothing contained in this section shall be held to apply to the Panama Canal.